TRANSACTIONS OF THE SECTIONS. 59 
On certain Deviations of the Plumb-line from its Mean Direction, as observed 
in the neighbourhood of Shanklin Down, in the Isle of Wight, during the 
' progress of the Ordnance Survey. By W. Hopkins, F.R.S. 
The difference of latitude between Greenwich and the station of the Ordnance sur- 
veyors at Dunnose, on the north side of Shanklin Down, as determined by triangu- 
lation, was greater by 2'22 seconds than as determined by zenith sector observations. 
When, however, a new station was chosen on the south side of Shanklin Down, the 
difference of latitude, as determined by triangulation, was less by 3°09 seconds than 
it appeared to be when determined by the zenith sector. These discrepancies would 
be accounted for, if the mass intervening between the stations at Shanklin Down 
were sufficient to produce, by its attraction on the plumb-line, the observed devia- 
tions. The requisite calculations for proving the adequacy of this cause had not 
been made; the tendency, however, would necessarily be to produce effects of the 
same nature as those observed ; and the author thought it probable that the intensity 
of the attraction of the hill would be found sufficient to account for the pheno- 
mena. 
On Railway Sections made on the Line of the Great Western Railway, between 
Bristol and Taunton. By W. Sanovers, F.G.S. 
The general séction is made to the scale of 33 inches to the mile, or 1 to 1920; 
extending over a length of 45 miles. The colours are those of the Ordnance Survey. 
In four places the sections are enlarged fourfold, or 40 feet to the inch; and full 
details are given on the scale of 4 feet to the inch. The colours of the Ordnance 
Survey are taken as a basis, and others are introduced to describe local facts. The 
railway passes first through the junction beds of red marl and lias; then for six 
or seven miles through new red sandstone, interrupted only at one place, where 
a tunnel, 324 feet in length, pierces the upper beds of the carboniferous limestone ; 
then for about twelve miles chiefly over alluvial tracts, separated by cuttings of new 
red sandstone’ beds, also touching the southern margin of the Nailsea coal-field. 
At twenty-one miles occurs the Uphill cutting, passing through new red sandstone 
and lias, and then carboniferous limestone, at the base of which is seen some masses 
of trappean rock. The railway is then carried along an alluvial plain of seventeen 
miles in length, interrupted only in one place, at three miles north of Bridgewater, 
by a deep cutting through new red marls and lias at Puriton. From the termina- 
tion of the alluvial tract to Taunton, the course is over a moderately level country 
of new red sandstone, and little occurs to attract the attention of the geologist, ex- 
cept the occasional appearance of diluvial gravel, which is seen to contain an in- 
creasing proportion of the killas as the observer approaches Taunton. There are 
_ four enlarged drawings. 
It is desirable to notice first, that called the Ashton cutting, where the carboni- 
ferous beds are displayed in three places. Over the two smaller masses and on the 
sides of the larger one are seen the usual boulder conglomerate, consisting of large 
and small fragments of limestone, partially water-worn, cemented by the ordinary 
red clay. As the superincumbent strata are carried over the two small masses of 
limestone, and apparently tend to ascend over the rock at the tunnel, it might be 
_ supposed that a local elevation had taken place subsequent to the deposit of the new 
red sandstones. This msy have been the case, but the inclination and curving of 
the beds may have been original; and I incline to this opinion, since, on the pre- 
sumption of the limestone having been elevated by violent force arising from igneous 
action, the conglomerate adjoining the limestone would have been pierced and thrust 
aside, and greater displacement manifested in the marly clays above. 
At the Uphill section the evidence differs: here not only are violent dislocations 
of the red marls and lias produced, so that the lias beds dip at an angle of 70° or 
more towards the plane of intersection between the lias and the limestone, which 
plane itself dips in the same direction ; but, in addition, the igneous rock appears in 
full force ; and it clearly bears relation not only to the fault, which brings into juxta- 
position the limestone and lias, but to an extensive fault in the limestone itself, by 
which the whole series of beds on one side differs from that on the other. 
On the nature of the trap rock and its metamorphic influence on the limestone, 
